The receptor tyrosine kinase Met, and its ligand, hepatocyte growth factor (HGF, also called scatter factor), have been implicated in promoting invasive growth of many tumor types due to inappropriate activation of Met function (Jankowski et al., 2003; Nardone et al., 2003; Trusolino and Comoglio, 2002; Birchmeier et al., 2003). This activation can arise from a variety of sources, but in each case the Met receptor activates signaling cascades that normally function to organize groups of cells into branching, tubular structures that are present in a variety of organs (Montesano et al., 1992; Sonnenberg et al., 1993; Rosen et al., 1994; Trusolino and Comoglio, 2002; Zhang and Vande Woude, 2003). The Met receptor plays a unique role during development as a master switch, which can stimulate proliferation and motility necessary for the full program of growth and scattering of cells. Its role in the invasiveness of many cancers makes it an attractive target for therapeutics (Ma et al., 2003). However, many questions remain about how the ligand, HGF, binds to Met and induces its tyrosine kinase cascade and thus leads to a biological response.
The Met receptor is part of a larger family of growth factor receptors with identical domain architecture that includes the Ron and Sea receptors (Monsin et al., 1992, Huff et al., 1993). The extracellular portions of Met family members are composed of three domain types. The N-terminal 500 residues fold into a Sema domain, which shares sequence homology with domains found in the Semaphorin and plexin families of neural development proteins (Winburg et al., 1998). As reported recently, Sema domains form a 7-bladed β-propeller structure (Antipenkov et al., 2003, Love et al., 2003). Met undergoes proteolytic cleavage within the Sema domain during normal processing, although the role for this remains unclear since cells that are unable to cleave Met show normal levels of Met activation upon ligand binding (Komada et al., 1993). A PSI domain, a small domain spanning about 50 residues and containing 4 disulfide bonds, follows the Sema domain. In addition to the Met receptor family, PSI domains are also found in the plexins, Semaphorins and integrins, hence its name (Bork et al., 1999). In Met, the PSI domain is connected via 4 IPT domains to the transmembrane helix and the kinase domain in the intracellular portion of the receptor. IPT domains are related to immunoglobulin-like domains and are named after their presence in plexins and transcription factors (Takagi et al., 1995).
HGF is a large growth factor of 728 residues that is produced as an inactive single-chain precursor which is proteolytically processed to form the biologically active disulfide-linked α/β-heterodimer (Nakamura et al., 1989; Hartmann et al., 1992; Kataoka et al., 2003). The α-chain folds into an N-terminal domain (N-domain) followed by 4 Kringle domains. The β-chain starts with residue Val495 and is homologous to the protease domain of chymotrypsin like serine proteases, which, like HGF, are activated by a proteolytic cleavage event (Perona and Craig, 1995; Hedstrom 2002). However, no protease activity has been demonstrated for HGF β-chain (Lokker et al., 1992) consistent with the absence of the key serine and histidine residues that are part of the ‘catalytic triad’ Asp[c102]-His[c57]-Ser[c195] ([chymotrypsinogen numbering]) required for catalytic activity in serine proteases.
Comparisons of the biologically active, two-chain HGF, and the inactive single-chain HGF precursor on the Met receptor have shown that both forms of HGF bind Met with similar affinity, but only the cleaved, mature form of HGF is able to activate Met (Lokker et al., 1992). In addition, various C-terminally truncated fragments of the α-chain (termed NK1, NK2, or NK4 depending on the number of Kringle domains retained) bind Met; in many cases they act as potent antagonists of Met receptor function (Cioce et al., 1996; Chan et al., 1991; Date et al., 1997). Studies involving the cross-linking of Met receptors by a variety of specific antibodies to its extracellular domain have demonstrated that simple dimerization of Met is sufficient for activation (Prat et al., 1998). Based on these characteristics, the fundamental mechanism for Met dimerization remains unclear.
Currently, there is no detailed structural information about HGF β-chain complexed with Met receptor. A completely solved crystal structure of the HGF β-chain complexed with Met receptor is needed, for example, for assays for Met-ligand (e.g., HGF β-chain) interaction and function, modeling the structure-function relationship of Met and other molecules, diagnostic assays for mutation-induced pathologies, and rational design of agents useful in modulating Met or HGF activity or activation.